Study Finds Bird-Flu Virus Can Spread Among Cats

By LAWRENCE K. ALTMAN

Published: September 3, 2004

The avian influenza virus that has spread widely among poultry and other birds in Southeast Asia and infected some people there has also crossed another species barrier to infect cats, and can be spread among them as well, Dutch scientists have found.

The finding is ''extraordinary because domestic cats are generally considered to be resistant to disease from influenza A virus infection,'' like that of the avian strain, the researchers are reporting in today's issue of the journal Science.

In the Dutch study, some cats with the infection died of it, while others survived. A few did not even show any symptoms that they were carrying the disease.

Whether cats can transmit the virus strain, A(H5N1), to humans is not known. The World Health Organization has received no reports that cats played a role in afflicting the 35 people who have developed A(H5N1) infection, all in Thailand and Vietnam, said Dick Thompson, a spokesman for the agency in Geneva. Those cases were traced chiefly to direct contact with sick birds.

Even so, the Dutch study has important implications for human and animal health, said Juan Lubroth, a senior animal health officer at another United Nations agency, the Food and Agriculture Organization.

The findings, Dr. Lubroth and the study's authors said, underscore a need to investigate the possible role of cats and an array of other animals in the spread of avian influenza among poultry farms and to humans.

An estimated 200 million birds have either died of A(H5N1) or been slaughtered to control the outbreak since last winter, when the strain simultaneously appeared in eight Asian countries. United Nations officials have described the scale of the epidemic -- geographically and economically -- as unprecedented for an avian flu outbreak.

The strain has also been particularly lethal for humans, killing 25 of the 35 people infected.

Many influenza experts and health officials fear a worst-case occurrence in which a person becomes infected with both an avian influenza virus and a human one. Under such a circumstance, the viruses might swap genes, creating a new virus that could cause an epidemic all over the planet much like that of the so-called Spanish flu of 1918-19, which killed 675,000 people in the United States alone and more than 20 million around the world.

The laboratory in Rotterdam that reported the new findings has conducted research on A(H5N1) since 1997, when its scientists detected the strain in a child who had died of the disease in Hong Kong.

The Hong Kong case was a scientific bombshell, because it was the first in which a new avian influenza virus had been transmitted from birds to humans without first mixing with mammalian influenza strains in pigs.

Since then, the A(H5N1) virus has mutated to become more virulent.

Last January a clouded leopard died, apparently of avian influenza, at a zoo in Thailand after eating virus-infected chickens, Thai health officials recalled in recent interviews in Bangkok.

A month later, scientists identified the A(H5N1) virus in three dead cats, and in a white tiger that recovered after becoming ill in the same zoo where the leopard died. The cats belonged to a Thai woman who had 15 in all, 14 of which apparently died of avian flu, although the remains of only those 3 could be found for testing. The woman did not develop bird flu.

Tests showed that the molecular makeup of the viruses isolated from the cats and the tiger was the same as that of the virus found in chickens.

After learning about those infections, the Rotterdam team, led by Dr. Thijs Kuiken, conducted three laboratory experiments by using the A(H5N1) virus isolated from a Vietnamese patient who had died of it. The findings confirmed what had been observed in the cats in Thailand.

First, Dr. Kuiken's team introduced the Vietnamese virus into the airways of three European shorthair cats, the breed generally used in animal experiments. All three became sick beginning the next day, and one died on the sixth day of illness. In comparison, none of three cats infected with the most common type of human influenza virus became ill.

In the second experiment, three cats were fed infected chicken. Examination of their tissues under a microscope showed that all three had developed severe lung damage similar to that seen among birds and humans. (People are not vulnerable to infection by eating chicken that is cooked, but the person who cooks it may be at risk from handling it, health officials say.)

In the third experiment, the researchers put two healthy cats in the same cage two days after infecting a third cat. The healthy cats also became ill.

Dr. Kuiken said in telephone interviews that he did not know whether these two cats had caught the infection by licking, through droplets or through the air. His study, he said, was not devised to determine how the cats spread the virus.

Additional research is needed because of the small size and scope of the Dutch study, experts said.

But the Food and Agriculture Organization ''is not set up to conduct this type of research,'' Dr. Lubroth said, adding that scientists at universities and other research institutes would have to do much of it, though with technical advice from his agency.

One avenue of research will be to test whether cats that are susceptible to other strains of influenza virus can spread those strains as well.

In addition, Dr. Kuiken said his team planned to test whether the original A(H5N1) virus, from the 1997 Hong Kong case, could infect cats, or whether only the later, mutated form could do so.

At the same time, Dr. Lubroth said, agricultural workers need to educate farmers about good practices like not raising swine with chickens.

Another reform will be teaching farmers to keep cats away from poultry, although that step, Dr. Lubroth said, ''may be as difficult as herding wild cats.''